The Fourdrinier process of paper making involves a succession of phases. Initially a slurry of cellulose fibers in water is distributed on a screen and some of the water is drained off. A web is formed which undergoes a press treatment in which the web is transported by a felt or a succession of felts to pass sinuously around a series of cylinders. Nip rollers in the press section force the felt against the formed web which, in turn, is pressed against the cylinders, to extract water mechanically. In current practice, the web leaving the press section contains from 35 to 45% solids. The web is then carried by felts into a dryer section consisting of heated cylinders, in which the water content of the web is reduced by evaporation to roughly that of the finished paper.
Afterdryers and calenders often follow the dryer section, ending with the reel. The dryers and afterdryer sections may contain 60 or more heated cylinders. A felt is used to hold the paper firmly against many of the heated dryer cylinders, for assuring contact of the web with the heated surface and thereby promoting drying efficiency.
In the dryer, the temperature of the first series of cylinders is comparatively low but it increases along the series of cylinders; the following series of cylinders effect a constant rate of drying. In this region the cylinders' temperature is uniform. At the end of the drying section, the temperature of the steam in the successive cylinders is increased to 187.degree. C. (370.degree. F.), for example, and the drying rate declines progressively. It is complicated and expensive to provide steam at a pressure such that a specified high temperature is maintained in each of the cylinders. This is especially true when temperature changes are to be made.
Steam-heated cylinders are massive, both because of their large size and substantial wall thickness. They ewe usually made of gray cast iron for economy, and their walls are quite thick; e.g., 25 mm. to 51 mm.(1" to 2") or more, to withstand the high internal steam pressure. A web may be 7.6 m. (25 ft.) wide, requiring corresponding long cylinders. The web may travel at 1000 m./min. (3300 ft./min.), or roughly 60 km./hr. (37 miles/hr.). That speed is impressive. By any standard, the capital investment in a paper making machine is huge, and a considerable amount of space is needed.
Various types of paper making apparatus differ from that outlined above. For example, the "Yankee" type is characterized by inclusion of one very large diameter dryer cylinder; e.g., a diameter of 3.6 m. to 5.5 m. (12 ft. to 18 ft.). There, the wall thickness is particularly great, to withstand the pressure of the contained high-temperature steam and to allow for periodic grinding to restore surface smoothness.
The highest temperature of any steam-heated cylinder is limited by the corresponding pressure of steam that can be safely contained within the cylinder. The maximum internal steam temperature of a dryer cylinder is approximately 187.degree. C. (370.degree. F.) because of the steam pressure limitation. It has been widely recognized that higher regulated temperatures, if feasible, would accelerate the drying process and would reduce substantially the number of dryer cylinders required. This is achieved here by replacing a large number of steam-heated cylinders in the more common type of paper making machine by a smaller number of novel cylinders heated to higher temperatures. Alternatively, using higher-temperature cylinders pursuant to the invention, a paper making machine having many cylinders could be operated at much higher speed, for greatly increased output.
Paper machine drying sections, worldwide, are almost universally heated by steam under pressure. Accordingly, it is appropriate to consider such apparatus in further detail, as a basis for appraising the advance in the art represented by the present invention.
As noted above, the temperature of a drying cylinder in a paper making machine is not determined by that which would be desirable from the point of view of performance, but by the limitations of cylinders heated by steam under pressure. This is evidenced by the large numbers of drying cylinders required in high-speed paper making machines or by the limited machine speed with lower temperature cylinders performing the drying function. Cylinders heated by steam under pressure have other significant limitations.
It is virtually impossible to regulate the temperature of a cylinder wall from point-to-point along its length, for developing a desired temperature profile across the width of a web being dried. Complicated, cumbersome arrangements have been proposed in an effort to compensate for increasing or declining temperatures at the margins of the web. However, no way has been found for adjusting the temperature profile of a cylinder heated by steam under pressure.
The external surface of a steam-heated cylinder responds slowly to an adjustment in steam pressure. This slow response time is manifested, for example, by the many minutes needed to bring the paper making machine from a cold start to full-speed operation. It is also manifested by the delayed change of a cylinder's external temperature in response to an adjustment in steam pressure.
The foregoing and other characteristics of a paper making machine whose dryer cylinders are heated by steam under pressure are impaired by some of the traits of the cylinder wall. Transfer of heat from the steam to the outer surface of the cylinder which contacts the web is impeded by many factors, including:
a) The considerable thickness of the cylinder wall needed for containing steam under the high pressure corresponding to the steam temperature, noting that the actual wall thickness is greater by a safety factor of 2.8 than that theoretically required for withstanding the steam pressure; PA1 b) The poor heat conductivity of gray cast iron, the customary metal chosen for the cylinder wall, rather than a more expensive metal of superior heat conductivity; PA1 c) A layer of condensate that forms and is distributed by centrifugal force over the cylinder's interior; PA1 d) A layer of scale that develops over the cylinder's internal surface; and PA1 e) A temperature drop required to extract heat from the steam, by condensation.
The difference between the temperature of the steam and that of the cylinder's external surface represents a waste of energy.
The enormous mass of the cylinder wall and the high inertial load require a large value of installed horsepower capacity and a correspondingly high energy cost to drive the machine.
The above factors that impede energy transfer, plus the thermal inertia of the massive cylinder wall, contribute to a long response time of steam-heated cylinders. The same factors limit the speed and productivity of the machine.
Recognition of the problems and limitations of steam as the heat source in dryers of paper making machines has prompted proposals of alternative heating media.
It has been proposed that dryer cylinders in paper making machines should be heated internally by electric power; but electricity is inordinately expensive.
It has also been proposed that a dryer cylinder for paper making apparatus should be heated by a flame within the cylinder. Transfer of heat from the gaseous combustion products to the cylinder requires extensive areas of metal exposed to the hot gases and requires efficient removal of the combustion products after their heat has been extracted, so as to provide necessary space that is to receive newly emitted gaseous exhaust. See Hemsath, U.S. Pat. No. 4,693,015 issued Sep. 15, 1987 and Calhoun U.S. Pat. No. 4,498,864 issued Jun. 6, 1961.
Still further, U.S. Pat. No. 4,688,335 issued Aug. 25, 1987 to Krill et al discloses use of a gas-fired radiant heat generator to heat a cylinder that acts on a web of fibers being pressed against the cylinder by a felt and a nip roller, the web having a large water content. The heater of Krill et al is in the form of a ceramic fiber matrix shaped as a cylindrical shell. An air-fuel mixture is supplied to the interior of the shell. The mixture burns as it emerges everywhere from the shell, heating the matrix. Unlike Hemsath, above, the energy of combustion in Krill et al is intended to produce radiant heat. The heated web-engaging cylinder in Krill et al operates at 315.degree. C. to 427.degree. C. (600.degree. F. to 800.degree. F.), being so hot that some of the free water that is present in spaces between the fibers of the web is converted to steam, which blasts other free water through and out of the web. This process is called "impulse drying". Even though the supplied air-fuel mixture is adjustable, reduction of the air-fuel supply is limited by the lowest rate needed to sustain combustion. Noting that the type of heater used in Krill at al to produce radiant heat is in the form of a complete cylinder, the heat output would almost certainly be excessive for use in the usual drying section of a paper-making machine, even with its air-fuel supply adjusted downward to a minimum. Moreover, if the temperature of the cylinder were reduced by adjusting the supply of air-fuel mixture for developing a suitable operating temperature at full-speed operation of the apparatus, little if any latitude of downward adjustment would be available for realizing still lower cylinder temperatures as is required during slowed operation of the apparatus.
Despite alternatives that have been proposed for heating the dryer cylinders of apparatus for making paper, steam under pressure continues to be the generally accepted heating method.